2021 Oklahoma-Texas Super Outbreak
The 2021 Oklahoma-Texas Super Outbreak was the second-largest, costliest, and deadliest tornado outbreak in United States history. The outbreak mainly affected the southern United States, in particular Oklahoma and Texas, with additional tornadoes occurring in Arizona, New Mexico, and Colorado on April 25th and 26th. The outbreak produced a total of 223 tornadoes in 5 states, with the states of Oklahoma and Texas being the most severely affected. April 27 was by far the most active day of the outbreak, producing 182 tornadoes, including 7 which were rated EF5, and 19 which were rated EF4. A total of 1,485 people were killed by the outbreak, the deadliest ever recorded, with an additional 32 people killed by powerful hailstorms (First time since 2000), lightning, and flash flooding. 1,997 of the tornado-related deaths were the result of 5 violent tornadoes in Oklahoma and Texas on April 27th. Several other tornadoes caused death tolls into the double digits, particularly in Oklahoma, Texas, New Mexico, and Colorado. Due to its extent, severity, and loss of life, the outbreak was labelled "Apocalyptic Alley" by both meteorologists and mass media. Meteorological synopsis The outbreak was caused by a vigorous upper-level trough that moved into Arizona and Utah on April 25. An extratropical cyclone developed ahead of this upper-level trough between Central New Mexico and Southern Colorado, and moved northeastward. Conditions were similar on April 26, with a predicted likelihood of severe thunderstorms, including the extended threat of strong to violent long-track tornadoes throughout the afternoon and evening hours; mixed-layer CAPE values were forecast to be around 3000-4000 J/kg, around West New Mexico and Texas. The storm mode on April 26 was predicted to include mostly discrete tornadic supercells during both the afternoon and the early evening, shifting over to a mesoscale convective complex, with more of a threat of damaging winds and hail during the nighttime hours. As the system moved eastward toward Oklahoma and Texas on April 27, a very powerful 80-100 knot mid-level jet stream moved in behind the trough and created strong wind shear, along with a low pressure center moving quickly northeastward across those areas on April 27. During the afternoon of April 27, CAPE values were estimated to be in the range of 2000-3000 J/kg across western New Mexico and Texas, with the moderate instability moving northeastward across Texas; additionally, temperatures across the southern United States ranged from the 70s to the lower 90s. Helicity levels ranged from 450-600m2/s2, which supported some significant tornadic activity and strong to violent long-track tornadoes. A total of 54 severe weather watches were issued by the Storm Prediction Center (SPC) over those 4 days in the outbreak area. This included 41 tornado watches-10 of which were Particularly Dangerous Situation (PDS) watches-and 13 severe thunderstorm watches. The SPC assigns numbers to each severe weather watch issued starting at the beginning of each year; the organisation unsuccessfully used two of their allocated watch numbers during this outbreak. April 25 A reasonably large area of possible severe storms for April 25 as the Storm Prediction Center (SPC) issued a slight risk of severe weather during the day, centered over Colorado and New Mexico. At 3:25pm CDT, the SPC issued a Particularly Dangerous Situation (PDS) tornado watch for much of Colorado and New Mexico. By the evening hours of April 25, a few tornadoes had been reported across the 2 states, one of which caused significant damage in Colorado. An intense supercell thunderstorm tracked near the Interstate 25 and a tornado emergency was declared for the city of Pueblo. A large EF2 wedge tornado struck the town, subsequently causing severe damage and killing 3 people. A total of 6 tornadoes and 3 tornado-related deaths were confirmed on the 25th. April 26 An enhanced risk of severe weather was issued for April 26th for portions of New Mexico and Texas along the border as conditions became even more favourable for extreme weather. A large PDS tornado watch with high possibilities for tornadoes for that same area that afternoon. Widespread tornado warnings were then issued for that area later that evening. A surface low corresponded to an area of upper-level divergence ahead of the downstream shortwave. As the low formed across New Mexico and deepened while moving east, a tightening pressure gradient force further strengthened the low level jet, therefore creating a broad warm sector across the southern states. This also generated stronger wind shear, providing better organisation for the supercell storms as a result. Numerous tornadoes touched down along the Texas-New Mexico border. Most of these tornadoes were weak, but a few of them caused some damage. A long-tracked wedge tornado caused EF2 damage across rural portions of Texas and New Mexico. An EF3 destroyed structures and caused severe damage at Roswell, New Mexico as well. A total of 22 tornadoes were confirmed on the 26th, although no fatalities occurred. April 27 Significant severe weather was ongoing early April 27 and continued for the entire calendar day virtually unbroken. The SPC issued a high risk of severe weather for the southern United States. Later that morning, the SPC even increased the probability for tornadoes to 45 percent along a corridor from Ardmore, Oklahoma, to, Norman, Oklahoma, an extremely rare issuance exceeding the high risk standards. Conditions became increasingly favourable for tornadoes during such an extreme tornado outbreak. Morning Squall Lines During the early morning hours, a cold front with several embedded low pressure areas extended from West New Mexico northeastward into Illinois. An upper level disturbance that had moved across the frontal boundary the previous evening sparked an area of thunderstorms that morphed into a squall line. This line of severe thunderstorms would produce tornadic activity from the evening of April 26 into the late morning of April 27. Early in the morning the squall line, packing straight line winds and numerous embedded tornadoes, moved across the border before proceeding to affect North Texas and South Oklahoma. The line strengthened as it moved through Texas and Oklahoma, partially due to a high amount of low-level moisture from the Gulf Of Mexico and increasing wind shear. A majority of tornadoes embedded in the initial squall line were weak, though several were strong and as such caused considerable damage. An EF3 tornado caused major damage to homes in Amarillo, Texas, and EF2 and an EF3 produced severe damage and a fatality near Wichita Falls, Texas. Another EF3 tornado resulted in heavy damage in downtown Lawton, Oklahoma, which was struck by a violent EF4 tornado later that afternoon. One embedded cell that began in Stephens County, Texas produced a long-tracked EF2 tornado that struck the town of Liberty, killing one person. That cell would then produce over 10 tornadoes (most rated EF1) in Jack County and another EF1 in Montague County. The initial storms caused widespread power and telephone line outages across Texas and Oklahoma. This line of storms also caused some NOAA weather radio transmitter sites to stop functioning for the remainder of the outbreak. Because of this, thousands of customers were without power and had no warning of any approaching tornadoes later that day. From the late morning to early afternoon, another squall line moved through northwestern parts of Texas and southwestern parts of Oklahoma and high wind shear and low-level moisture persisted. However, this time several discrete supercells developed along and in front of the line, spawning several weak tornadoes across Franklin, Hopkins, and Lamar counties in northwestern Texas around noon that day Afternoon supercells